The invention pertains to a system for controlling the brake systems of at least two wheels of a vehicle.
The state of the art offers many different methods for preventing the wheels from locking by intervening in the brake systems of the wheels. To do this, an instability criterion is usually derived from the deceleration and slip of the wheels. When the brake pressure is now controlled at the individual wheels in such a way that each wheel experiences the optimum deceleration independently of all of the other wheels, it is possible under so-called ".mu.-split conditions" for the vehicle to spin. A .mu.-split condition of this type is present when the vehicle is traveling on a surface such that the friction values on the right side of the vehicle are significantly different from those on the left. In this case the wheels which are moving on the part of the driving surface with the higher coefficient of friction are braked much more strongly than the wheels moving on the surface with the lower coefficient of friction. To prevent the vehicle from spinning in this case, a so-called "yaw moment buildup delay" is carried out, so that, in cases where the coefficients of friction of the driving surface are asymmetric, the difference in the braking force between the wheels of one axle of the vehicle does not become too pronounced. In principle, during the delay of the buildup in the yaw moment, the increase in pressure at the wheel most recently showing a tendency to lock, i.e., the so-called "high wheel", is limited when the other wheel, i.e., the so-called "low wheel", starts showing a tendency to lock. Various solutions for delaying the buildup of yaw moment like this are known. For example, when the low wheel is showing a tendency to lock, the pressure at the high wheel can continue to be built up but on a reduced gradient. It is also known that, in cases where the low wheel is showing a tendency to lock, the brake pressure at the high wheel can be kept constant until the brake pressure starts to build up again at the low wheel. It is known from U.S. Pat. No. 4,852,009 that, during travel around a curve, the increase in the brake pressure at the rear wheels is reduced when an instability occurs at the front wheel first showing a tendency to lock. The reduced pressure increase is selected as a function of the magnitude of the current transverse acceleration. In this way, the function of the yaw moment buildup delay is improved during travel around a curve. It is also known from DE-OS 39 25 828 that the difference between the brake pressure at the high wheel and that at the low wheel of one axle can be monitored, so that, as soon as a predetermined nominal pressure difference is exceeded, the brake pressure at the high wheel can be kept constant or reduced. This predetermined nominal pressure difference is varied as a function of the coefficient of friction at the low wheel.
In the SAE technical paper "VDC, The Vehicle Dynamics Control System of Bosch" Advancements in ABS/TCS and Brake Technology, SP-1075(1995) a system is presented in which essentially the angular yaw velocity of the vehicle is measured and compared with a nominal value derived from the longitudinal velocity of the vehicle and the steering angle. When the measured angular yaw velocity exceeds the corresponding nominal value, the yaw behavior of the vehicle can be modified and thus the driving stability increased by means of wheel-specific overbraking or underbraking.
The object of the present invention is to optimize the braking behavior under .mu.-split conditions.